A printing system for generating the various colors that can be produced on a two-primary color printer, each color being produced by the combination of two bit maps, the ON bits of which specify the pixels to be printed in each primary.
To add highlight color to a typical black and white print, a xerographic printer can be designed with a tri-level development process. The photoconductor is charged in the form of an image where some areas are charged positively and some areas are charged negatively relative to non-image areas. Then, by using black toner with a positive potential and a primary color toner with a negative potential, both black text and colored highlight areas can be printed on the same print in one pass. These highlight areas can be regular shapes such as a colored borders or highlighted areas of the page, or more complex shapes such as a rough approximations of figures such as flowers.
For the remainder of this document, it will be assumed that one of the primary colors is black, but this is not necessary. The process is equally workable for a combination of two other colors.
Colored inks are typically produced in primary colors, and for this discussion we will assume that they are red, green and blue. Then, the printer, at any given moment, will have black toner and one toner of one of these three primaries. It is left to the user to load any primary, and to change to another primary at any time by removing the original toner cartridge and inserting another.
Most printers typically print characters in a solid color, usually black. In a tri-level printer it would be equally typical to print text in either solid black or solid color. To print variations of color that can be produced from two primaries is difficult. Starting with black and red ink, for instance, the operator can specify a number of matrices, each containing any number of pixels and then assign the colors red and black to the pixels to create a range of colors from light pink to dark red. However, the specification of each pixel in a matrix for each color is a time consuming process. In addition, for each color, the resultant matrix must be printed out to check its color, texture and density, and redone if the print is not as desired. Also, a number of palettes, each having as many as 100 colors, is typical. A fast and systematic method of generating colors is required.